I am a novice in electronics and to this board, so please forgive me when I trip over myself here.

I am building a small car that I want to "bump start". Effectively, I will have a very small DC motor directly driving the real axle and would like to have the motor switch on, once it starts turning (i.e. like a bump start).

I understand that a DC motor should generate a current if it is mechanically turned. I would like that current to activate a timed circuit that then drives the motor for X seconds (or until it is reset by some other mechanism). First, is this possible? And, second, can it be done with an analog circuit?

I'm stepping out of my area of expertise here but I couldn't resist. Attached is a circuit that may or may not start you off in the right direction.

By "bumping" the motor, the generated voltage fires the gate on the first transistor which allows current through to the gate on the second transistor which allows current through to the motor. (You'll have to "size" all components appropriately.)

Stopping the motor from spinning manually would cease current flow to the gate on the first tranisitor thus stopping current flow to the second transistor (I think).

I think that the transistors are going to get rather warm. A comparator might detect bump V, capacitivly coupled to a 555, which would further forward bias comp. keeping output low untill 555 timed out, shutting every thing down- hopefully, might just oscillate.

I am about to demonstrate my newbie(ness). When you say comparator, could you be a little more specific?

I am attaching the schematic for the motor control circuit I am envisioning. Because I am planning on driving this little motor with a 9V (it is only rated for 1.5 - 3V), I am going to decrease the duty cycle to try to keep it from burning up, and limit the "run time" to 2-3 seconds (however long it takes to get down the track). R4 and R5 will actually be pots that can be adjusted to control run time and duty cycle. S3 will trigger the circuit and would be where I'd expect to "bump start" from. And S4, will probably be attached to a phototransistor or something that would kill the motor once it passes under the light at the finish line. Finally, where I have the indicator LED D4, I would have the motor.

Will the piggy-backed transistors pick up the extremely small voltage I am looking to trigger at? If I use a comparator, could you guide me to an appropriate component?

It pulls almost 2.5A getting up to speed, and once at speed it pulls 1.8A. Is that too high for the 555/6 circuit?

I expect it will only need to run a dozen (or less) times, for no more than 5 seconds per run. So, burning out components is ok, if they survive 20??? times.

Jeremy

Click to expand...

You will need a p-channel MOSFET to drive the motor, and CMOS 555s.
I don't understand what you are doing with the oscillator. Are you planning to PWM the motor?
What are you planning to use for the 9V power supply?

You will need a p-channel MOSFET to drive the motor, and CMOS 555s.
I don't understand what you are doing with the oscillator. Are you planning to PWM the motor?
What are you planning to use for the 9V power supply?

Click to expand...

The oscillator was intended for PWM. And planning to use a standard 9V (alkaline) battery.

A voltage comparator compares two voltages, if one is higher than the other, output is high, & vis-a vers, the difference can be a fraction of a milli V. My choice would be 4 Ni-Cd AAs for battery. Direct drive might give verry slow acceloration, so gear reduction might be necessary- also giving bigger bump V- if bump is hard enough. Sorry, but it's late.

I had not even tried running the motor with a lower voltage until you sent this schematic. I can run it off of as little as 1.5V with speed getting as high as I need it. Acceleration is not a problem since the track starts on a decline (this is an "open class" Pinewood Derby). I will try putting this circuit together this evening and report on the success. Thank you!

A possible complication: Hopefully bump polarity will be correct, +, when car goes forward; my test sais yes.
What are some of the rules? Do not skimp on power, the RS motor is not known for great torque, according to user comments.

Bump polarity is positive. I've got a 5V power source, which gets the wheel up to full speed in < 0.5 second. Add gravity, and I am not worried about acceleration. I've almost got the circuit complete on the breadboard and will test it shortly.

OK, so I got the circuit together and it works! However, there are a couple issues. First, the moment I put power to the circuit, the transistors start getting hot. Second, Once it bump starts, the current flowing through the motor is only about 200mA. If I connect the motor directly to the 5V power source, it draws over 3A initially, and settles down to about 2.2A.

Are these limitations of the components (transistors, comparator (NTE922), 555 timer, etc.), or the circuit in general? What values do you suggest for the non-labeled resistors and the cap? And, finally, can you detail the connections at the timer? I don't have anything going to the discharge or threshhold pins . . . am I supposed to?

Post your schematic, with all component values.
I don't see how it could work. The specs on NTE922 say it won't. I would use LM393.
Your PNP needs to be able to handle stall current. You have to provide enough base current to saturate it at stall current. This may be as much as 300mA, which has to come from the other transistor (which was drawn as a PNP, but should be NPN).
I would dump the PNP and use a logic level PMOS transistor.